Thermal switch



May 26,, 1942. w. c. SMITLEY THERMAL SWITCH Filed-Sept. 20, 1959 Inventor". Wayrwe C.Srnitle\ b X/Wc. His Attorney.

Patented May 26, 1942 THERMAL SWITCH Wayne C. Smitley, Cleveland Heights, Ohio, assignor to General Electric Company, a corporatin of New York Application September 20, 1939, Serial No. 295,801

8 Claims.

My invention relates in general to thermal switches, and in particular to switches employing a bimetallic element to make and break the interengagement of the switch contacts. Still more particularly, my invention relates to thermal switches of the glow discharge type in which the heat for actuating the bimetallic element is generated by a glow discharge in an ionizable medium. Such thermal switches are particularly applicable for use in connection with the starting and operation of electric discharge devices such as, for instance, fluorescent lamps of the well known type now in commercial use.

Certain types of electric discharge devices, such as the aforementioned fluorescent lamps, employ a pair of thermionic electrodes which require a certain amount of preheating before the arc will strike therebetween. For this purpose, it is customary to interconnect the two electrodes in series across the line by means of an auxiliary switch, one form of which is a thermal switch of the glow discharge type referred to above. After the thermionic electrodes of the discharge device have been preheated to discharge sustaining temperature, the thermal switch automatically breaks the series connection of the electrodes, thus causing the arc to strike therebetween. Such switches must therefore provide a positive time-delayed circuit interruption in order to insure the preheating of the lamp electrodes. In addition, they must be capable of an exceedingly great number of makes and breaks, somewhat commensurate with the great number of times the lamps operated by such switchareapt to be turned on and off.

In the glow switches heretofore employed for starting electric discharge devices of the fluorescent lamp type, the full lamp electrode heating current passed between the switch electrodes during the time the switch functioned as a glow discharge device. This full heating current, however, passing through the glow discharge, sputtered off the activating material on the switch electrodes in a very short. time, so that the surfaces of such electrodes were quickly damaged and the switch life materially foreshortened.

One object of my invention is to provide a thermal switch of the glow discharge type having a relatively long life.

Another object of my invention is to provide a thermal switch of the glow discharge type having means for limiting the current flow passing through the glow discharge over that normally passing through the switch contactswhen closed.

A feature of my invention is the incorporation of a high resistance element or resistor in the glow switch which 'is connected. in series with the activated switch electrode. This resistance element or resistor serves to regulate or limit the glow discharge current without limiting the current which passes through the switch contacts when closed. Thus, the full lamp electrode preheating current is still available for the required preheating of the lamp electrodes and the proper starting of the lamp.

Further objects and advantages of my invention will appear from the following description of species thereof and from the accompanying drawing in which:

Fig, 1 is an elevation, partly in section, of a thermal switch comprising my invention on an enlarged scale; Fig. 2 is anelevation of a modified form of thermal switch comprising my invention; Figs. 3 and4 are longitudinal sectional views of two further modifications of my invention; and Fig. 5 is a diagrammatic illustration of the circuit employed for sta'rting and operating, by means of a thermal switch according to the invention, an electric discharge device having a pair of thermionic electrodes.

Referring to the drawing, the thermal switch I0 according to the invention comprises an el0n-. gated hermetically sealed envelope or bulb II preferably of glass and containing a filling of an ionizable gas or mixture of gases at a suitable pressure. This gas filling'preferably consists of neon or argon at a pressure of approximately 25 to mm. of mercury.' One end of the envelope' II is provided with an inwardly extending stem I2 through which leading-in wires I3, I4 extend into the interior of the envelope. A base I5, comprising a shell I6 and a bottom center contact I'I secured together by suitable insulation I8, is secured to the stem end of the envelope in any suitable manner, such as by cement IS. The said base is preferably of the bayonet type, although it may be of the screw-threaded or any other suitable type. The leading-in wires I3 and I4 are connected to the shell I6 and center contact I I, respectively, of the base.

A hollow, preferably cylindrical tube of metal 20, such as nickel, is mounted substantially centrally within the envelope I I so as to extend longitudinally thereof. The said tube is supported from the stem I2 by a support wire 2I sealed into the stem. The tube 20 serves as one of the electrodes during the time the switch functions as a glow discharge device. For this purpose, the tube is provided with a coating of a suitable activating or electron-emissive material, such as' barium. Extending centrally through the tube 20 is a bimetallic element or strip 22 which is secured at one end to leading-in wire l4. Like the tube 20, the bimetallic element 22 is also coated with a layer of suitable activating or electron-emissive material. Thus, the bimetallic element 22 serves as the other electrode during the period the switch functions as a glow discharge device. The other or free end of the bimetallic element 22 is provided with a switch contact member 23 preferably in the form of a short length of wire bent into a more or less V-shape. This wire contact member is mounted on the bimetallic element so as to lie in a plane extending transversely of the envelope Ii. The inner portion 24 of leading-in wire [3 extends through the envelope II, in spaced relation to the electrode tube 20, to a point adjacent, and in the path of movement of, the vertex of the V-shaped contact 23. Thus, the inner lead 24 serves as the other or stationary contact member for the switch. If desired, the inner portion 24 of lead ing-in wire l3 may be formed as a separate piece fastened in any suitable manner, such as by welding, to the leading-in wire l3.

To prevent the disintegration of the activated electrode surfaces of the switch Ill during the continuance of the glow discharge therein, a high resistance element or resistor is connected in series with the electrode tube 20 in accordance with the invention. This resistor may be of any suitable form and may be located either within the envelope H, within the base I6, or entirely without the switch. Thus, as shown in Fig. 1, the resistor may be in the form of a wire coil 25 connected at opposite ends to the leadingin wire l3 and the support wire 2| of the electrode 20, and surrounding the bimetallic element 22 at a point adjacent the base or supported end thereof. The resistor in this case has the additional function of a supplemental heater for the bimetallic element 22, i. e., the heat radiated by the wire coil 25 supplements the heat from the glow discharge. As a result of this supplemental heating action of the wire coil 25, the time required for the closing of the switch contacts 23, 24 is somewhat decreased over that required where only the heat from the glow discharge is relied on to heat the bimetallic element 22.

Instead of surrounding the bimetallic element 22, the resistor 25 may be simply mounted within the envelope II in spaced relation to the bimetallic element 22, as shown in Fig. 2. Alternatively, the resistor 25 may be mounted in the base l6 the switch, as shown in Fig. 4. In the latter case, the support wire 2| for the tubular electrode is extended through the wire seal portion of the stem l2 in order to enable the connection of one end of the wire coil resistor to such support Wire.

In the modification shown in Fig. 3, the resistor is in the form of a carbon ring 26 mounted within the base I5, preferably within the tubular portion of the stem l2 and surrounding the conventional exhaust tube 2'! extending centrally through said stem tube. As in the previous forms of the invention, the carbon ring resistor 25 is connected across the leading-in wire l3 and the support wire 21 for the tubular electrode 20.

In the forms of the invention shown in Figs. 3 and 4, the portion of leading-in wire !4 extending between the seal portion of the stem and the bottom contact I! of the base may be covered, if desired, with suitable insulation, such as an insulating sleeve, in order to thereby prevent accidental contact, in the limited space within the stem tube, of such leading-in wire with the resistor 25 or 26.

Fig. 5 illustrates diagrammatically the circuit preferably employed for starting and operating, by means of a glow discharge thermal switch according to the invention, an electric discharge device 28 having a pair of thermionic electrodes 29 and 30, such as the fluorescent lamp now in commercial use. One end of electrode 23 is connected by lead 3! to the bimetallic element 22 and switch contact 23, while one end of the other lamp electrode 3i is connected by lead 32 to the other switch contact 24. The other ends of the two lamp electrodes 29 and 36 are connected to opposite sides of a source of alternating current such as the ordinary volt 60 cycle commercial sources, electrode 39 being connected by lead 33 to one side of the line and electrode 29 being connected to the opposite side of the line through lead 34, a ballast 35 (preferably an inductance or choke coil), lead 36, a main switch 31, and lead 38.

When the main switch 3'! is closed, the potential across electrodes 20 and 22 within the thermal switch breaks down or ionizes the gas filling therein, thus causing a glow discharge to take place between the switch electrodes 20, 22. However, the high resistance element or resistor 25 in series with the tubular electrode 20 prevents the full lamp electrode heating current from passing through the glow discharge. This limitation of the glow discharge current by the resistor 25 therefore materially reduces the sputtering of the electron-emissive material off the activated switch electrodes 20, 22. Consequently, damage to the switch electrode surfaces is prevented and the life of the switch is greatly prolonged.

The continuance of the glow discharge within the switch Ill quickly heats up the bimetallic element 22, thus causing movement of the same towards the stationary contact member 24. The glow discharge continues until the contact 23 at the end 01 the bimetallic element 22 engages the other contact 24. The closure of the switch contacts immediately short circuits the glow discharge path between the switch electrodes so that the glow discharge is instantly terminated and further heating of the bimetallic element 22 discontinued. Since the resistor 25 is electrically connected between electrode 20 and the stationary switch contact 24, it forms, together with the glow discharge, a parallel circuit to that through the switch contacts 23, 24. Consequently, the closure of the switch contacts, in addition to short circuiting the glow discharge path, also short circuits or cuts out the resistor 25 so that the full lamp electrode heating current, which is necessary in order to obtain proper heating of the lamp electrodes as well as proper starting of the lamp, can pass through the closed switch contacts 23, 24 and consequently through the lamp electrodes 29, 30. It is therefore evident that, by the particular arrangement of the resistor 25 in the circuit in accordance with the invention, the current passing through the glow discharge is regulated so as to prevent damage to the glow discharge electrodes and prolong the life of the switch, without limiting in any man ner the current which passes through the switch contacts. Thus, the resistor does not interfere in any way with the proper starting of the arc discharge in the lamp 28.

Because of the slight lag in the movement of the bimetal behind the temperature rise, and further because of the tendency of, the switch contacts 23, 24 to weld together, the said contacts, once; they have closed, remain in enga ement with each other for an extended period of time of from approximately one-half to one and one-half seconds or thereabouts. Such a more or less delayed opening of the switch l therefore enables the current flowing through the circuit to adequately preheat the lamp electrodes. 29, 30 to the temperature at which they become sufliciently electron-emissive to support an arc discharge therebetween. Consequently, upon the opening of the switch contacts 23, 24, the arc discharge immediately strikes between the electrodes 29, 30 of the lamp, thereby shunting the thermal switch ID. The are discharge occurs in the lamp 28 in preference to a restarting of the glow discharge in the thermal switch for the reason that the breakdown potential of the gas within the lamp 28 is somewhat less than the breakdown potential of the gas within the switch It, being approximately 50 to 60 volts for the lamp as compared to approximately '75 to 90 volts for the switch.

Inasmuch as the arc discharge within the lamp 28 shunts the thermal switch [0, the latter is virtually out of the circuit so that no current flows through the switch. Consequently, the bimetallic element l9 returns to its normal position, and the switch 10 is then ready to restart the lamp even while the arc discharge yet continues within the lamp 28. It is thus evident that the restart time of the thermal switch It! is substantially the same as the original starting time, which is a highly desirable feature inasmuch as it permits the lamp to be turned off and on again without any appreciable delay in the restarting of the arc discharge within the lamp. In this connection, it has been found that the start and restart time of the glow discharge switch l0 according to my invention, 1. e., the total elapsed time between closure of the main switch 31 and actual starting of the arc discharge within the lamp 28, is approximately two and one-half to three seconds or very close thereto.

Although I have shown and described various species of the thermal glow switch comprising my invention, it should be understood that other modifications are possible within the scope of the invention. Thus. the resistor 25, instead of being mounted within the switch, may be mounted. entirely without the same. In such case, however, three base terminals are required in order to enable the proper connection of the resistor into the circuit. If desired. the stationary contact member 24 may also be made of bimetal arranged to move in the same direction as the bimetallic element 22 thus providing ambient temperature compensation for the switch contacts.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A thermal switch comprising a hermetically sealed envelope containing an ionizable gaseous filling, an electrode mounted within said envelope, a second electrode comprising a bimetallic element mounted within said envelope in close proximity to but spaced from said firstmentioned electrode, one'of saidelectrodes having a coating of electron-emissive material thereon, said bimetallic element being supported at one end only and being provided at its other free end with a switch contact, a stationary contact member within said envelope extending into close proximity to and disposed in the path of movement of said switch contact, and a high resistance element electrically connected to and between said first-mentioned electrode and said stationary contact member.

2. A thermal switch comprising a hermetically sealed envelope containing an ionizable gaseous filling, an electrode mounted within said envelope, a second electrode comprising a bimetallic element mounted within said envelope in close proximity to but spaced from said first-mentioned electrode, one of said electrodes having a coating of electron-emissive material thereon, said bimetallic element being supported at one end only and being provided at its other free end with a switch contact, a stationary contact member within said envelope extending into close proximity to and disposed in the path of movement of said switch contact, and a high resistance element disposed within said envelope and electrically connected to and between said first-mentioned electrode and said stationary contact member.

3. A thermal switch comprising a hermetically sealed envelope containing an ionizable gaseous filling, an electrode mounted within said envelope, a second electrode comprising a bimetallic element mounted within said envelope in close proximity to but spaced from said firstmentioned electrode, one of said electrodes having a coating of electron-emissive material thereon, said bimetallic element being supported at one end only and being provided at its other free end with a switch contact, a stationary contact member within said envelope extending into close proximity to and disposed in the path of movement of said switch contact, and a high resistance element disposed within said envelope and electrically connected to and between said first-mentioned electrode and said stationary contact member, said high resistance element being disposed closely adjacent the supported end of' said bimetallic element so that a substantial proportion of the heat radiated by said high resistance element is received by said bimetallic strip.

4. A thermal switch comprising a hermetically sealed envelope containing an ionizable gaseous filling, an electrode mounted within said envelope, a second electrode comprising a bimetallic element mounted within said envelope in close proximity to but spaced from said firstmentioned electrode, one of said electrodes having a coating of electron-emissive material thereon, said bimetallic element being supporting at one end only and being provided at its other free end with a switch contact, a stationary contact member within said envelope extending into close proximity to and disposed in the path of movement of said switch contact, and a high resistance element within said envelope closely surrounding the supported end of said bimetallic element and electrically connected to and between said first-mentioned electrode and said stationary contact member.

5. A thermal switch comprising a hermetically sealed envelope contaiing an ionizable gaseous filling and having a stem at one end thereof, a base secured to the stem end of said envelope, and having terminal members, an electrode mounted within said envelope, a second electrode comprising a bimetallic element mounted within said envelope in close proximity to but spaced from said first-mentioned electrode, one

of said electrodes having a coating of electronemissive material thereon, said bimetallic element being supported at one end only and being provided at its other free end with a switch contact, a stationary contact member within said envelope extending into close proximity to and disposed in the path of movement of said switch contact, aid bimetallic element and said stationary contact member being electrically connected to the terminals of said base by leading-in wires extending into said envelope through said stem, and a high resistance element disposed within said base and electrically connected to and between said first-mentioned electrode and said stationary contact member.

6. A thermal switch comprising a hermetically sealed envelope containing an ionizable gaseous filling and having a stem at one end thereof, said stem including a tubular portion and an exhaust tube extending substantially centrally therethrough, a base secured to the stem end of said envelope, and having terminal members, an electrode mounted within said envelope, a second electrode comprising a bimetallic element mounted within said envelope in close proximity to but spacedfrom said first-mentioned electrode, one of said electrodes having a coating of electronemissive material thereon, said bimetallic element being supported at one end only and being provided at its other free end with a switch contact, a stationary contact member within said envelope extending into close proximity to and disposed in the path of movement of said switch contact, said bimetallic element and said stationary contact member being electrically connected to the terminals of said base by leading-in wires extending into'saidenvelope through said stem, and a high resistance element disposed within said base and electrically connected to and between said first-mentioned electrode and said stationary contact member, said high resistance element comprising a ring of carbonaceous material disposed within the tubular portion of said stem and surrounding the said exhaust tube therein.

'7. A thermal glow switch comprising an envelope containing an ionizable medium and a pair of electrodes between which a glow discharge occurs upon application of a potential thereto, one of said electrodes being a thermo-sensitive element, a contact member located adjacent to said thermo-sensitive electrode and adapted to engage with said electrode upon initiation of said glow discharge, and a high resistance element electrically connected to and between the other electrode and said contact member so as to be in series with the discharge gap between said electrodes to appreciably reduce the current flow therebetween but being shunted by the engagement of said thermo-sensitive electrode and said contact member.

8. A thermal glow switch comprising an envelope containing an ionizable medium and a pair of electrodes between which a glow discharge occurs upon application of a potential thereto, one of said electrodes being a bimetallic element, a contact member located adjacent to said bimetallic electrode and adapted to be engaged thereby upon initiation of said glow discharge, and a high resistance element electrically connected to and between the other electrode and said contact member so as to be in series with the discharge gap between said electrodes to appreciably reduce the current flow therebetween but being shunted by the engagement of said bimetallic electrode and said contact member.

WAYNE C. SMITLEY. 

